Lathes typically employ chucks to removably attach a workpiece to the rotating shaft of the lathe. Several different types of chucks are known, the most popular being the so-called "4-jaw independent chuck" which has 4 reversible jaws, each of which may be independently adjusted, and the so-called "3-jaw universal chuck" which has 3 jaws which move simultaneously and automatically center the work. A removable key, commonly know as a "square end wrench", is typically used to tighten or loosen the chuck at the start or conclusion of the work. Such lathe chucks and such removable keys are well known in the art.
On occasion, an operator will forget to remove the key from the lathe chuck after loading the workpiece into the lathe and tightening the lathe chuck. Subsequent operation of the lathe causes the key to be forcibly ejected from the rotating chuck under centrifugal action, thereby endangering the operator, other bystanders, and/or the machine itself.
Similar situations can occur with other rotationally operative devices which employ chucks and removable keys, such as drills, drill presses and the like.
The foregoing safety problems have been recognized in machine shops and technical training and vocational schools where industrial shop safety is a major concern. Various safety key devices have been suggested to insure automatic disengagement of the key from the chuck when the key is released by the user. See, for example, U.S. Pat. Nos. 1489976, 2215621, 2387981, 2388414, 2442717, 2598119, 2609719, 2618995, 2660081, 2690690, 2807732, 3174365, 3457813, 3504580, 3673895, 3686985, 3728038, 3869943, 3977686, 4085943, 4111079, 4157836, 4324512, and 4395171, U.K. Patent Applications Nos. 2069379A and 2157204A, French Patent No. 679601, German Patent No. 2259649 and Swiss Patent No. 141368.
Unfortunately, all of the foregoing safety key devices are believed to suffer from one or more disadvantages. Many of these devices suffer from the disadvantage that they require an operator to continually exert pressure axially downward as the safety key device engages the chuck so as to allow a leading tip of the key to project outward from the remainder of the key and thereby prevent the safety key from ejecting itself from the chuck during use. Continually exerting pressure axially downward during rotation of the safety key device may require unnatural body movement by the operator which can be inconvenient. The inconvenience of having to maintain this awkward axially downward pressure during use is believed to have caused many prior art safety key devices to fall into disuse.